1. Field of the Invention
The present invention relates to an apparatus, a method, and a computer program product for monitoring biological information of a target person.
2. Description of the Related Art
Various methods are known for monitoring autonomic-nervous activity of living beings such as a person. Recently, the frequency-analysis method is being commonly used. In the frequency-analysis method, levels of sympathetic and parasympathetic nervous activities are calculated by analyzing fluctuations in the heartbeat intervals.
A heart rate of a person is usually calculated from peak-to-peak intervals in a waveform obtained from an electrocardiogram, a photoelectric, or pressure-type of sphygmograph of the person. A peak appears in the waveform due to a heartbeat. In the frequency-analysis method, a series of data on heartbeat intervals are collected, the data are converted into a frequency spectrum, and powers of a low-frequency band (LF: a band ranging from 0.05 hertz to 0.15 hertz) and a high-frequency band (HF: a band ranging from 0.15 hertz to 0.4 hertz) in the spectrum are determined as the heart rate. In this manner, the frequency-analysis method makes use of the relation between the LF and the HF and the autonomic-nervous activity.
The frequency-analysis method, however, requires high-end computer environment, which is not always available. If such a computer environment is not available, one approach to analyze the frequency is to use indexes such as the RR50 and the mean of the sum of the squared differences in successive R-R intervals (MSSD).
The RR50 is an index that is obtained as follows. That is, difference between intervals of two adjacent heartbeats is calculated for all the heartbeats in a predetermined period, for example, 1 minute, and the RR50 is the number of heartbeats for which the difference exceeds 50 milliseconds.
On the other hands, the MSSD is an index that is obtained as follows. That is, square of difference between intervals of two adjacent heartbeats is calculated for all the heartbeats in a predetermined period, and the MSSD is the average of the squares. Both the RR50 and the MSSD have been used to monitor autonomic-nervous activity and they make use of degree of fluctuation in the heartbeat intervals.
Autonomic-nervous activity can be monitored with even other techniques. There is known a method of evaluating a parasympathetic-nervous activity in real time based on an electrocardiogram and a breathing patter of a person (see JP-A 2005-319256 (KOKAI)). On the other hand, there is known a method of evaluating parasympathetic-nervous activity based on magnitude variation among the last three data of the heart-rate intervals of a person (see JP-A H8-117198 (KOKAI)).
A person's emotional status, that is, whether the person is feeling stress, can be determined by monitoring the autonomic-nervous activity. Moreover, this technique can be effectively used to prevent a person from snoozing when he/she is determined to be sleepy, and determine a state of sleep based on a changing pattern of the autonomic-nervous activity. This technique can be used even for medical purposes. For example, JP-A No. 3643564 (KOKAI) discloses a technique in which an autonomic-nervous function is evaluated based on a fluctuation of the heart rate, and changes of the heart rate is monitored before and after a blood vessel gets compressed due to a cuff during a blood-pressure measurement.
However, the conventional frequency-analysis method involves a large amount of calculations so that this method cannot be realized on low-end computer systems that are generally incorporated in mobile devices. On the other hand, high-end computers systems are expensive and require high-power batteries, which adds to the cost.
With regard to the RR50 and the MSSD indexes, the result includes both low-frequency components (equivalent to the LF) and high-frequency components (equivalent to the HF) those are a part of fluctuation components so that it is difficult to separate the result into those for sympathetic nerve and those for parasympathetic nerve.
The technique disclosed in the JP-A 2005-319256 (KOKAI) has a disadvantage that signals from two pieces of information are required, i.e., the electrocardiogram and the breathing pattern, so that the structure of the overall system becomes complicated.
The technique disclosed in the JP-A H8-117198 (KOKAI) has a disadvantage that because the evaluation is made based on the number of predetermined patterns of last three data, for example, “large, small, large”, or “small, large, small”, the evaluation is not quantitative.